| 1. |
- Chou, Chia-Ying, 1990-
(författare)
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Integrated Computational and Experimental Study of Additively Manufactured Steels
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The design freedom Additive Manufacturing (AM) offers provides new solutions for improving functionality in industrial applications. It also offers unique opportunities when it comes to materials design.Powder Bed Fusion – Laser Beam (PBF-LB) is currently one of the most popular commercial AM techniques for metallic materials partly due to the relatively low surface roughness and the large design flexibility. However, the number of materials suitable for the PBF-LB process is still rather low and to accelerate the development of grades tailored for this AM process, dedicated computational tools for alloy design are needed. Of importance for materials design, is computational thermodynamics and kinetics coupled with CALPHAD materials descriptions since it enables calculations for multicomponent materials making it possible to predict the effect of varying composition.In this thesis, computational thermodynamic and kinetics coupled with materials characterization are applied to study the microstructure evolution during PBF-LB. Two material classes are in focus – hot-work tool steels and ferritic stainless steels. For the hot-work tool steel, the cooling rates during PBF-LB processing are high enough to induce martensite transformation and in the as-built microstructure a martensitic matrix is observed and some fraction of retained austenite. A solidification sub-structure due to micro-segregation during printing is also observed. Solidification calculations are performed to predict the micro-segregation showing agreement with experimental measurements. The segregation results are then used as input to a semi-empirical martensite start temperature model making it possible to explain the location and amount of retained austenite.In addition to compositional optimization, a computational framework for AM alloy design needs to include the possibility to tailor the AM post heat treatments. An alternative to the conventional hardening treatment is thus studied in the current work. A model for precipitation kinetics is combined with experimental characterization to explore the effect of tempering on the as-built microstructure in comparison to the tempering effect on an austenitized microstructure. The results show that the precipitation kinetics is strongly dependent on the starting structure and that direct tempering of the as-built microstructure changes the precipitation sequence compared to the conventional heat treatment route. The calculations reproduce this result suggesting that it is a thermodynamic effect stemming from different matrix compositions.The other material class, the ferritic stainless steels, is studied in terms of its response to post-heat treatments. The as-built microstructure is characterized by high dislocation density and a fine grain structure in some cases as well as a solidification sub-structure. The mechanical properties of the as-built material are in general good for these steels, however, stress relieving is most often a required post process for the as-built components which may decrease the mechanical properties. To maximize the gained benefits from the unique process condition of PBF-LB, simulations are applied to study the possibility of post heat treatment optimization.To construct a computational framework for AM materials design, multiscale modeling capabilities are needed. This work shows the value of computational thermodynamics and kinetics for understanding the materials behavior on the microscale and hence, contributes to the construction of such a framework. By understanding the physical metallurgy, and enable modeling of the AM processes, the industrialization of AM can be accelerated.
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| 2. |
- Liu, Jianling, 1992-
(författare)
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Phase separation in duplex stainless steel: characterization and mitigation
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Duplex stainless steel (DSS) is a category of widely used stainlesssteel, which are strategically important in a variety of applications such asin the food industry, chemical engineering plants and nuclear power plants,due to their attractive combination of mechanical properties and corrosionresistance. However, these steel grades are sensitive to the so-called ‘475°Cembrittlement’, because of a phase separation (PS) phenomenon in theferrite phase, which decomposes into Fe-rich ferrite (α) and Cr-rich ferrite(α'), when exposed to temperatures within the miscibility gap. The PS isaccompanied by an increase of micro-hardness of the bcc phase and a severeloss of toughness, leading to the deterioration of the mechanical properties.Therefore, the upper service temperature of DSSs in industrial applicationshas been limited to around 250° C.In the present work, the PS in DSSs was mainly investigated by smallangleneutron scattering (SANS). A quantitative analysis method based onthe SANS data was proposed to evaluate both the wavelength and amplitudeof PS in Fe-Cr based alloys, e.g. DSSs. The wavelength and amplitudequantified by this methodology showed a good agreement with the resultsby atom probe tomography (APT). Then, SANS measurements and thismethod were applied for a comprehensive investigation on the applicationof DSSs and their weldments under industrially relevant conditions, i.e.low/intermediate temperature and prolonged aging time, in order to pursuethe structure-property relation. The current study shows that the measuredCr amplitude is connected to the change of micro-hardness and impacttoughness. Moreover, embrittlement is a function of both the isothermalaging temperature and time.In order to find effective ways to mitigate the PS, this work has alsoattempted to investigate the governing thermodynamics and the kinetics ofPS. First, the effects of alloying elements (e.g. Ni, Al, Co) on the Fe-Crsystem were critically reviewed since such understanding could pave theway for the design of the next generation of DSSs that are less susceptibleto embrittlement; Second, the process route can also influence the PS andit was therefore investigated. It was found that a faster cooling rate aftersolution treatment leads to the lower rate of PS. Moreover, applying anexternal magnetic field may also affect the kinetics of PS: an in-situ SANSstudy showed that a 1.5 T applied magnetic field can significantly delay thePS of DSSs in the early stages.This thesis can be summarized in two parts: i) demonstration of SANSfor characterizing and quantifying PS in DSSs under industrially relevantconditions; ii) discussion of the possibility to mitigate PS in DSSs to becomeless susceptible to low/intermediate temperature embrittlement.
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| 3. |
- Xiang, Shengmei, 1991-
(författare)
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Oxidation, Creep and Fatigue Synergies in Cast Materials for Exhaust Manifolds
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The future development of engines of heavy-duty vehicles towards reduced CO2 emission will increase the exhaust gas temperature and render the exhaust atmosphere more corrosive. The current service material of exhaust gas components - a ferritic ductile cast iron called SiMo51 - will soon meet its upper-temperature limit. Three alternative materials were suggested in a previous study: SiMo1000 (ferritic, nodular cast iron), D5S (austenitic, nodular cast iron), and HK30 (austenitic, cast steel). Together with SiMo51 (reference) the alternative materials are investigated in the present thesis with respect to performance and degradation mechanisms, under the individual or collective influence of high-temperature fatigue, corrosion, and creep.Firstly, fatigue, corrosion and corrosion-fatigue at 800oC were studied to establish their degradation mechanisms and relative performance. The individual influence of fatigue and corrosion was studied using low-cycle fatigue (LCF) tests in argon, and oxidation tests in a synthetic exhaust gas (5%O2-10%CO2-5%H2O-1ppmSO2-N2(bal.)), respectively. The collective influence of fatigue and corrosion was studied using LCF test in the synthetic exhaust gas. The degradation mechanisms were analyzed through extensive characterization of the tested specimens. Different crack-initiation mechanisms were found for the various combinations of materials and atmospheres. In argon, crack initiation was generally caused by secondary phases at the surface (graphite in SiMo51/SiMo1000, graphite and intermetallics in D5S) and near-surface casting defects (in all materials). In the exhaust atmosphere, crack initiation was generally influenced by oxide intrusions (formed from oxidized graphite in SiMo51 and expressed as dendrite boundary corrosion in HK30), internal fracture of intermetallics (in D5S), decarburization creating microcracks/stress concentrations (in SiMo1000), and near-surface casting defects (in all materials). The relative performance was analyzed using fatigue and oxidation curves.Secondly, two improvements were attempted for SiMo1000, a modified casting geometry for improved graphite morphology and a surface treatment method, nitrocarburizing. The first attempt resulted in significantly reduced decarburization, decreased initial crack size formed by graphite/matrix debonding and an improved corrosion-fatigue life of 8 to 16 times. The second attempt resulted in two types of microcracks after the process and a self-sustained growth of the diffusion layer, when subjected to high-temperature corrosion. A strong corrosion-fatigue synergy was found, reducing the fatigue lifetime by 84-89%.Thirdly, the collective influence of fatigue and creep was studied for D5S using regular LCF tests (reference) and creep-fatigue tests, with either tension or compression dwell. Both dwell directions reduce fatigue life but promote different creep-fatigue-corrosion interactions. Tension dwell produces a clear creep-fatigue synergy, generating creep pinholes near graphite nodules. Typically, such damage is observed in regular creep tests of several months. Compression dwell decreases lifetime more than tension dwell due to increased peak tensile stress and a more pronounced surface crack initiation by an oxide wedging mechanism.The investigation in the present study gives a better understanding of the correlation between microstructure and corrosion-fatigue/creep-fatigue properties in materials used for exhaust gas components. Moreover, the combination of fatigue tests in argon/exhaust atmosphere, oxidation tests in the exhaust atmosphere, creep-fatigue tests, and creep tests from a previous study shows how corrosion, fatigue, and creep individually and synergistically affect the material performance at elevated temperature.
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| 4. |
- Xu, Xin, 1987-
(författare)
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Phase Separation in Stainless Steels Studied by Small-angle Neutron Scattering
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fe-Cr based steels, i.e. stainless steels, possessing a combination of excellent corrosion resistance and good mechanical properties, have indispensable applications ranging from low-end cooking utensils, to sophisticated components for nuclear power plants. However, the bcc/bct phase containing stainless steels which have a miscibility gap (MG) suffer from the so-called “475 oC embrittlement” leading to hardness increase and toughness deterioration. It occurs due to demixing of Fe and Cr leading to the formation of Fe-rich (α) and Cr-rich (α′) regions in bcc/bct phases. The demixing is referred to as phase separation (PS).The goal of this work was to study PS in ferrite containing stainless steels mainly by small-angle neutron scattering (SANS). Firstly, the application of different experimental techniques for the study of phase separation in Fe-Cr based steels was reviewed and supplemented by new measurements. SANS was shown to be very sensitive to the nanostructure change caused by PS and capable of characterizing the early stages of PS in Fe-Cr alloys. However, atom probe tomography and transmission electron microscopy are complementary to SANS. Therefore, in order to have a more complete view of the microstructure, the combination of these techniques should be pursued. Secondly, the factors affecting the initial microstructure prior to aging treatment and the effect of the resulted initial microstructure on PS were systematically investigated using binary Fe-Cr model alloys. The critical temperature of the MG was determined to be located between 560 and 580 oC in binary Fe-Cr. The results indicate that the solution treatment temperature above the MG and the cooling rate after solution treatment have significant effects on the initial microstructure and thus on PS during subsequent aging. The mechanisms responsible for the changed aging behavior are Cr clustering, quenched-in vacancy and decomposition during cooling. Therefore, computational simulations should take into account these factors and the initial microstructure to make predictions that are more accurate. Thirdly, the study was extended to PS in commercial duplex stainless steels (DSSs) which are of practical importance in various industries, e.g., nuclear power. It is found that alloying elements have an important effect on PS in DSSs. The grade 2507 (25 %Cr, 7 %Ni) experiences stronger PS than grade 2205 (22 %Cr, 5 % Ni) for the same heat treatment. Moreover, the fracture mechanisms as well as the mechanical properties depend on the extent of PS. Finally, the fundamental aspects regarding the neutron scattering behavior for Fe-Cr alloys were examined. The results show that the nuclear and magnetic scattering of neutrons depend on the evolution of the nanoscale compositional fluctuation in Fe-Cr alloys. The ratio of the magnitude of nuclear scattering versus magnetic scattering varies with the extent of PS.
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| 5. |
- Ma, Taoran
(författare)
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Powder-metallurgical processing and phase separation in ternary transition metal carbides
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ternary transition metal cubic carbides have high hardness and are potential carbides in cemented carbide and cermet tools, as well as hard coatings used to improve metal cutting performance. In the present work, (Ti,Zr)C, (V,Nb)C, and (V,Ta)C ternary cubic carbides were synthesized using traditional powder-metallurgical methods. The effect of synthesis temperature and starting materials on synthesis is investigated, and the microstructure evolution during aging is studied. (Ti,Zr)C was found to decompose into lamellae upon aging at the temperature range from 1150 to 1800 °C. A similar microstructure was observed in (V,Ta)C and (V,Nb)C- 0.5 wt% Fe. All of these structures were found to form through discontinuous precipitation.The grain misorientation distribution of (Ti,Zr)C aged at 1400 °C is investigated. It was found that decomposition tends to occur at high-angle grain boundaries above 25°. The hardness of as-synthesized (Ti,Zr)C powder was found to be 41±6 GPa. Fully decomposed (Ti,Zr)C particles were found to be slightly harder than the undecomposed counterpart. On the other hand, in (V,Nb)C-0.5 wt% Fe, the decomposed structure formed upon aging at 1200 °C was found to have a hardness of 26±2 GPa, which is basically the same as the unaged alloy.Furthermore, the sintering behavior of (Ti,Zr)C with WC-Co is investigated. There are two γ-phases in the final microstructure, one TiC-rich and one ZrC-rich. (Ti,Zr)C was found to decompose at an early stage of sintering, and the final grain size of WC and the two γ-phases was found to be 10% smaller than that in a reference WC-TiC-ZrC-Co composite.
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| 6. |
- Mukherjee, Deepjyoti, 1991-
(författare)
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Phase field modeling of precipitation reactions in miscibility gap systems
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As a backbone to the cutting tools and rock drilling industry, cemented carbides have been used widely due to their high hardness and wear resistance. Most commercially used cemented carbides contain a hard phase made of Tungsten carbide (WC) and a binder phase which is generally ductile. Lately, a secondary hard phase is desired by replacing W in WC partially with one or more metal substitutes such as Ti, V, Ta, Cr and Zr to better its mechanical properties. Some of these carbides are observed to exhibit unusual microstructures during ageing. For example, (Ti,Zr)C present along with WC, has been observed to undergo phase separation from a supersaturated phase, called $\gamma$, to Ti-rich and Zr-rich domains leaving behind an array of precipitates morphologically manifesting as lamellae. This phase separation process has been termed as discontinuous precipitation(DP) as it resembles the classical DP reaction observed in certain binary and multi-component systems. The behaviour comes from the presence of a miscibility gap in the carbide Ref. Borgh et al. 2014 and Ma et al. 2016 due to which the usual response of the system should be to undergo spinodal decomposition(SD), however, the carbide chooses a different path which questions the governing mechanism behind its decomposition process. Several factors are believed to affect such a process and one of such factors is the strain energy, which is generated due to the difference in lattice parameters of the separating phases. When compared to a different miscibility gap system, such as Fe-Cr, where the strain energy is quite low and the response is SD. Other factors such as grain boundary diffusion, atomic mobility, and gradient energy coefficient (κ) are also believed to have an effect on the decomposition process. Therefore, a thorough investigation of the factors is required and, a powerful tool to study the spatio-temporal evolution of the microstructure such as the phase field method, should be used.According to some experiments the lamellae are generally observed to nucleate at grain boundaries and later grow with the help of grain boundary migration Ref. Borgh et al. 2014. The moving grain boundary leaves behind a series of alternate strands of Ti and Zr rich phases. The growth mechanism behind the moving boundary is believed to be assisted by diffusion of solutes along the grain boundary and generation of the elastic strain energy by it. The phenomenon is commonly known as diffusion induced grain boundary migration(DIGM) and it is believed to be a key part of DP Ref. Hillert and Purdy 1977 and Chongmo and Hillert 1981. In order to recreate DIGM and DP an energetic coupling between the mole fraction and phase field variable is required so that it accounts for the generated strain energy during the process. The main focus of this thesis will be to develop a phase field model accounting for such coupling which will predict DIGM in binary systems and use it further as a medium to model DP in (Ti,Zr)C. The model for DP could be used to predict and control its formation as its occurrence prone to increase the hardness of the carbides. Therefore, it can be used as a tool to design alloys and develop better alternatives. The alternatives could be used to prevent DP in the carbides which could be done by using different metal substitutes that will prefer SD over DP or vice versa.
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| 7. |
- Salmasi, Armin, 1983-
(författare)
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ICME guided study of mass transport in production and application of cemented carbides
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cemented carbides are metallic composites consisting of a WC hard phase and a ductile binder, usually Co-based, produced by powder metallurgy and sintering. Cemented carbides are an essential part of modern material and manufacturing processes. However, Co powder is classified as a carcinogenic material with serious health hazards, and most virgin Co reservoirs are located in conflict regions. In addition, there are monopolies in the market for pure tungsten. Therefore, reducing the consumption of cobalt or replacing it with other non-hazardous elements would increase the sustainability of cemented carbide production. Furthermore, advances in production technology can help overcome raw material limitations. One such advancement is non-homogeneous structures and properties for optimization of microstructure which is the topic of this thesis. Integrated computational materials engineering (ICME) and its complementary tools, calculation of phase diagram (CALPHAD), and ab-initio modeling are strong tools that bridge experimentation and modeling. In this thesis, a framework for the material design of non-homogeneous cemented carbides is proposed and tested using these computational tools. The workflow of the material design of non-homogeneous microstructure and properties were studied on different length scales. Atomistic modeling with density functional theory (DFT), ab-initio molecular dynamics (AIMD), and generalized hydrodynamics (GHD) were used to model the viscosity of liquid Co binder. In addition, the mobility of Ti and Fe in disordered BCC TiFe alloy was assessed using new experimental data from the diffusion couple experiments and an electron probe micro-analyzer (EPMA). These two studies were conducted to complete the data necessary to study cemented carbides’ processing and performance. The other studied phenomenon studied by experimentation and modeling is the formation of the gradient zone and the γ cone structure. In addition, a phenomenological model for liquid phase migration (LPM) was created and implemented using the homogenization approach. The LPM pro- cess was studied experimentally and modeled with the YAPFI software. A study of these performers was necessary to link processing and microstructure. On the performance side, the chemical interaction between cutting tools and Ti alloys was studied in detail through experimentation and modeling of diffusion. In addition, hardness and toughness models were applied to predict the longevity of studied cemented carbides. Finally, by applying ICME and material design, a high entropy alloy (HEA) alternative to Co binder was designed, produced, and tested. The research presented in this dissertation attempts to fill the gaps in the material design workflow of cemented carbides by developing new tools and methods based on ICME and CALPHAD paradigms. This goal is achieved by studying different length scales, processing methods, microstructure, properties, and performance of cemented carbides.
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| 8. |
- Ullbrand, Jennifer
(författare)
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Phase field modeling of Spinodal decomposition in TiAlN
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- TiAlN thin films are used commercially in the cutting tool industry as wear protection of the inserts. During cutting, the inserts are subjected to high temperatures (~ 900 ° C and sometimes higher). The objective of this work is to simulate the material behavior at such high temperatures. TiAlN has been studied experimentally at least for two decades, but no microstructure simulations have so far been performed. In this thesis two models are presented, one based on regular solution and one that takes into account clustering effects on the thermodynamic data. Both models include anisotropic elasticity and lattice parameters deviation from Vegard’s law. The input parameters used in the simulations are ab initio calculations and experimental data.Methods for extracting diffusivities and activation energies as well as Young’s modulus from phase field results are presented. Specifically, strains, von Mises stresses, energies, and microstructure evolution have been studied during the spinodal decomposition of TiAlN. It has been found that strains and stresses are generated during the decomposition i.e. von Mises stresses ranging between 5 and 7.5 GPa are typically seen. The stresses give rise to a strongly composition dependent elastic energy that together with the composition dependent gradient energy determine the decomposed microstructure. Hence, the evolving microstructure depends strongly on the global composition. Morphologies ranging from isotropic, round domains to entangled outstretched domains can be achievedby changing the Al content. Moreover, the compositional wavelength of the evolved domains during decomposition is also composition dependent and it decreases with increasing Al content. Comparing the compositional wavelength evolution extracted from simulations and small angle X-ray scattering experiments show that the decomposition of TiAlN occurs in two stages; first an initial stage of constant wavelength and then a second stage with an increasing wavelength are observed. This finding is characteristic for spinodal decomposition and offers conclusive evidence that an ordering transformation occurs. The Young’s modulus evolution for Ti 0.33 Al 0.67 N shows an increase of 5% to ~398 GPa during the simulated decomposition.
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| 9. |
- Xiang, Shengmei, 1991-
(författare)
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High-Temperature Corrosion-Fatigue of Cast Alloys for Exhaust Manifolds
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The introduction of gas-driven Otto engine and the corresponding usage of bio-fuels in heavy-duty engines will render the exhaust atmosphere more corrosive and bring a higher working temperature to exhaust manifolds. The current service material, a ferritic ductile cast iron called SiMo51, will soon meet its upper temperature limit set by the ferrite-austenite transformation at 860ºC. Three alternative materials, as well as SiMo51 serving as reference, are investigated in the present thesis emphasizing on high-temperature corrosion fatigue. The first aim of this study is to obtain material data and give a quantitative ranking of the materials’ performance. Low-cycle fatigue (LCF) tests at 800ºC in a synthetic exhaust gas (5%O2-10%CO2-5%H2O-1ppmSO2-N2 bal.) are conducted to evaluate the materials’ performance in simulated real working scenarios, where high-temperature, corrosive atmosphere and fatigue conditions during testings are similar to the conditions experienced by the exhaust manifolds. To evaluate the individual effect from high-temperature fatigue and isolate the impact from corrosion, the materials are tested under the same settings but in an argon atmosphere. To evaluate the individual effect from high-temperature corrosion and isolate the impact from mechanical deformation, oxidation tests are carried out at 800ºC in the same synthetic exhaust gas. The second aim is to identify and understand different oxidation behavior and failure mechanisms in the materials, realized by considerable characterizations of the tested specimens.From the fatigue tests, it is found that the austenitic stainless steel HK30 has the highest fatigue resistance, followed by the austenitic cast iron Ni-resist D5S, and the ferritic ductile cast irons SiMo1000 and SiMo51, a ranking valid in both atmospheres. In the exhaust atmosphere, for instance, the improvement in fatigue strength at 15,000 cycles relative to SiMo51 are 260%, 194% and 26%, respectively. Different crack initiation and propagation mechanisms are found for the various combinations of materials and atmospheres. In the exhaust atmosphere, for instance, crack initiation is assisted by oxide intrusion in SiMo51 and crack propagation is affected by crack branching in HK30, mechanisms not observed in argon. By comparing the S-N fatigue curves in the two atmospheres, the influence of oxidation on fatigue life is evaluated. The fatigue life of the cast irons are surprisingly found to be higher in the exhaust atmosphere. Several explanations are suggested for this, considering their very different oxidation behaviors. This study provides accurate test data that can be used to help industry avoid over-dimensioned design. The investigation of the failure mechanisms promotes better understanding of the correlation between microstructure and mechanical properties. Moreover, the combination of fatigue tests in argon, fatigue tests in exhaust and oxidation tests in exhaust, shows how corrosion and fatigue individually and synergistically affect the materials’ performance at high temperature.
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| 10. |
- Borgh, Ida, 1983-
(författare)
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Aspects of Structural Evolution in Cemented Carbide – Carbide Size, Shape and Stability
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cemented carbide is a composite material used in applications like cutting tools and rock drilling inserts. The material commonly consists of WC grains embedded in a Co-rich binder phase and the material properties strongly depend on the WC grain size. Hence, to tailor the properties it is important to understand the fundamental mechanisms of grain coarsening. At the same time, the higher demands on material properties today also require new solutions. In the present work, some different aspects of structural evolutions in cemented carbides have been investigated.The first part of the work considers WC grain coarsening by means of size, size distribution and shape. Some efforts of the work have been to evaluate the effects of C-activity and initial WC powder size and distribution on the coarsening behavior in the material using different characterization techniques, e.g. scanning electron microscopy, and electron backscattered diffraction. Additionally, two earlier developed models are used and evaluated with the experimental data. The results indicate that the C-activity will affect size, size distribution and shape of the WC grains. It was also observed that the initial WC powder size and size distribution will have a large influence on the WC grain coarsening. The statistical shape was found to fit a spherical approximation but for individual grains both faceted and non-faceted shapes was observed. Steps and planar defects were observed supporting that the nucleation of new atomic layers is the main rate limiting mechanism for grain coarsening.The second part of this work considers the carbide phase stability in the (Ti,Zr)C system. The phase stability was investigated after synthesizing and aging a mixed (Ti,Zr)C using X-ray diffraction and different types of electron microscopy techniques. A decomposed lamellar structure was found with a composition variation of approximately 10% between the 50-75 nm thick lamellas. The experimental investigations were supported by computational work and the results were in good agreement. Additionally, two cemented carbide related systems were studied. A miscibility gap was found in the two investigated systems, (Ti,Zr,W)(C,N)-Co or Fe-graphite, and the effect of N2-gas pressure was investigated suggesting a critical N2-gas pressure below 0.1 bar.
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| 11. |
- Hou, Ziyong, 1986-
(författare)
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Study of precipitation in martensitic Fe-C-Cr alloys during tempering : Experiments and modelling
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding the precipitation reaction is very important since precipitation hardening is one of the most effective strengthening mechanisms in metallic alloys. In martensitic steels, a tempering heat treatment is often performed. During tempering various new phases are precipitated and the spatial and temporal evolution of these precipitates strongly influences the properties of the steel, such as strength/ductility, creep, fatigue and hot corrosion resistance. Therefore, the possibility of quantitative modelling of the precipitation process will provide many opportunities for advanced materials and process design and optimization as well as service life assessments. The Fe-C-Cr system forms the basis for tool steels and is consequently used in many applications such as e.g. metal forming operations. They are characterized by a high hardness and good toughness, even at elevated temperatures.In the present work, the as-quenched martensitic microstructures of four Fe-C-Cr alloys with varying Cr and C contents were characterized by Light Optical Microscopy (LOM) and Electron Microscopy. The effects of Cr and C on the morphology of martensite were investigated. It was found that Cr addition had a similar effect as C on the martensitic morphology and on the ratio of high-angle grain boundary (HAGB) to low-angle grain boundary (LAGB). However, the micro-hardness was unaffected by the Cr addition whilst it was strongly influenced by the C addition.In addition, a quantitative experimental characterization of the precipitates formed during tempering of the martensite was performed. The Langer-Schwartz theory combined with the Kampmann-Wagner-Numerical (KWN) method, as implemented in the software TC-PRISMA, was used to predict the precipitation of carbides after tempering in one of the model alloys: Fe-0.15C-4.0Cr (mass%). The microstructure characterization of the as-quenched material provided vital input parameters for the modelling work and a comparison was made between the modelling predictions and the experimental results. The effect of parameters such as dislocation density, grain size and interfacial energy on the precipitation of carbides was discussed.
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| 12. |
- Zhou, Jing, 1984-
(författare)
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Experimental study of phase separation in Fe-Cr based alloys
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Duplex stainless steels (DSSs) are important engineering materials due to their combination of good mechanical properties and corrosion resistance. However, as a consequence of their ferrite content, DSSs are sensitive to the so-called ‘475°C embrittlement’, which is induced by phase separation, namely, the ferrite decomposed into Fe-rich ferrite (α) and Cr-rich ferrite (α'), respectively. The phase separation is accompanied with a severe loss of toughness. Thus, the ‘475°C embrittlement’ phenomenon limits DSSs’ upper service temperature to around 250°C.In the present work, Fe-Cr binary model alloys and commercial DSSs from weldments were investigated for the study of phase separation in ferrite. Different techniques were employed to study the phase separation in model alloys and commercial DSSs, including atom probe tomography, transmission electron microscopy and micro-hardness test.Three different model alloys, Fe-25Cr, Fe-30Cr and Fe-35Cr (wt. %) were analyzed by atom probe tomography after different aging times. A new method based on radial distribution function was developed to evaluate the wavelength and amplitude of phase separation in these Fe-Cr binary alloys. The results were compared with the wavelengths obtained from 1D auto-correlation function and amplitudes from Langer-Bar-On-Miller method. It was found that the wavelengths from 1D auto-correlation function cannot reflect the 3D nano-scaled structures as accurate as those obtained by radial distribution function. Furthermore, the Langer-Bar-On-Miller method underestimates the amplitudes of phase separation.Commercial DSSs of SAF2205, 2304, 2507 and 25.10.4L were employed to investigate the connections between phase separation and mechanical properties from different microstructures (base metal, heat-affected-zone and welding bead) in welding. Moreover, the effect of external tensile stress during aging on phase separation of ferrite was also investigated. It was found that atom probe tomography is very useful for the analysis of phase separation in ferrite and the radial distribution function (RDF) is an effective method to compare the extent of phase separation at the very early stages. RDF is even more sensitive than frequency diagrams. In addition, the results indicate that the mechanical properties are highly connected with the phase separation in ferrite and other phenomena, such as Ni-Mn-Si-Cu clusters, that can also deteriorate the mechanical properties.
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